The mortars,prepared with various contents of fly ash,were immersed in 5% Na2SO4 solutions.Non-destructive testing,surface observation and ultrasonic test were used to track the changes of the surface cracks and the inner cracks of mortars attacked by sodium sulfate solution.Thermal analysis was used to identify the compounds and laser scanning confocal microscope(LSCM)and scanning electron microscope(SEM)were used to examine the microstructure of the mortars immersed in 5% Na2SO4 solution for 360 days.Results showed that the formation of ettringite was responsible for cracking, and the incorporation of fly ash could reduce the permeability and prevent the sulfate ingress.The surface and inner cracks were reduced by reducing the amount of ettringite, thus the effect was better with the increase of the dosages of fly ash.